Fluororubber-containing powder coating composition

ABSTRACT

A fluororubber-containing powder coating composition comprising a fluororubber, an adhesion-preventing material and optionally a cross-linking agent, which composition has an particle size of 10 to 2,000 μm and an apparent density of at least 0.2 g/cc, which provides a homogeneous smooth coated film having no or only a small amount of bubbles.

This application is a divisional of copending application Ser. No.08/066,933, filed on May 25, 1993 now U.S. Pat. No. 5,340,864, which isa Rule 62 Continuation of 07/701,344 filed May 9, 1991, which is a Rule62 Continuation of 07/481,184 filed Feb. 20, 1990, which is a Rule 60Continuation of 07/273,983 filed Nov. 21, 1988, all now abandoned, theentire contents of all of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fluororubber-containing powdercoating composition. More particularly, the present invention relates toa powder coating composition comprising a fluororubber, anadhesion-preventing material and optionally a cross-linking agent.

2. Description of the Related Art

A lining of a fluororubber can be formed on a substrate by variousmethods. For example, it can be formed by laminating a compounded sheeton the substrate followed by cross-linking, by applying a solution typecoating on the substrate and cross-linking the fluororubber or byapplying an aqueous emulsion type coating on the substrate andcross-linking the fluororubber. However, each of these methods has itsown drawbacks and it is highly desired to improve the methods forforming fluororubber linings onto a substrate.

In the case of the compounded sheet, the film has sufficient thicknessbut it cannot be applied on a substrate having a complicated shape.Although the solution or emulsion type coating can be applied on thesubstrate having the complicated shape, the formed film cannot be madesufficiently thick.

SUMMARY OF THE INVENTION

An object of the present invention is to provide afluororubber-containing coating composition which overcomes thedrawbacks of the above described conventional methods for formingfluororubber linings on a substrate.

According to the present invention, there is provided afluororubber-containing powder coating composition comprising afluororubber, an adhesion-preventing material and optionally across-linking agent, which composition has an particle size of 10 to2,000 μm and an apparent density of at least 0.2 g/cc.

DETAILED DESCRIPTION OF THE INVENTION

A "fluororubber" herein used is intended to define a fluorine-containingpolymer which has a glass transition temperature of not higher than 10°C. and rubbery elasticity at room temperature. In the present invention,any of the conventionally known fluororubbers may be used.

Specific examples of these fluororubbers are vinylidenefluoride/hexafluoropropylene copolymers, vinylidenefluoride/tetrafluoroethylene/hexafluoropropylene copolymers, vinylidenefluoride/chlorotrifluoroethylene copolymers,tetrafluoroethylene/propylene copolymers, hexafluoropropylene/ethylenecopolymers, perfluoro(alkyl vinyl ether) (including those having pluralether linkages)/olefin (e.g. tetrafluoroethylene, ethylene, etc.)copolymers, fluorosilicone polymers, fluorophosphazene polymers and thelike. Some of the fluororubber may have iodine atoms and/or bromineatoms bonded to polymer chains to increase a cross-linking reactivity ofthe rubber. Such fluororubbers are disclosed in Japanese Patent KokaiPublication No. 125491/1978, U.S. Pat. No. 4,243,770, Japanese PatentPublication No. 4115/1978, U.S. Pat. No. 4,035,565, Japanese PatentKokai Publication No. 2031/1984 and U.S. Pat. No. 4,501,869.

The fluororubber includes thermoplastic fluororubber. The thermoplasticfluororubber comprises at least one elastomeric polymer segment and atleast one non-elastomeric polymer segment, at least one of the polymersegments being a fluorine-containing polymer segment. Preferably, thethermoplastic fluororubber comprises the elastomeric polymer segment(s)and the non-elastomeric polymer segment(s) in a weight ratio of 40:60 to95:5.

A preferred example of the thermoplastic fluororubber is a fluororubbercomprising a polymer chain consisting of two or three polymer segments,an iodine atom bonded to one end of the polymer chain and a residuewhich has been formed by removing the iodine atom from an iodidecompound and is bonded to the other end of the polymer chain, one ofwhich segments (when the polymer chain consists of two segments) or oneor two of which segments (when the polymer chain consists of threesegments) is or are one or two elastomeric polymer segments having amolecular weight of 30,000 to 1,200,000 selected from the groupconsisting of (1) vinylidene fluoride/hexafluoropropylene ofpentafluoropropylene/tetrafluoroethylene (molar ratio of45-90:5-50:0-35) polymer and (2) perfluoro(C₁ -C₃ -alkyl vinyl ether)(including those having plural etherlinkages)/tetrafluoroethylene/vinylidene fluoride (molar ratio of15-75:0-85:0-85) polymer,

the remaining segment(s) is or are one or two non-elastomeric polymersegments having a molecular weight of 3,000 to 400,000 selected from thegroup consisting of (3) vinylidene fluoride/tetrafluoroethylene (molarration of 0-100:0-100) polymer and (4)ethylene/tetrafluoroethylene/hexafluoropropylene,3,3,3-trifluoropropylene-1, 2-trifluoromethyl-3,3,3-trifluoropropylene-1or perfluoro(C₁ -C₃ -alkyl vinyl ether) (including those having pluralether linkages) (molar ration of 40-60:60-40:0-30) polymer,

a weight ration of the elastomeric polymer segment(s) and thenon-elastomeric polymer segment(s) being from 40:60 to 95:5.

The preferred examples of the thermoplastic fluororubber are describedin Japanese Patent Publication No. 4728/1983 and U.S. Pat. No.4,158,678.

The coating composition of the present invention may be cross linked bya conventional cross-linking method, such as amine cross-linking, polyolcross-linking, peroxide cross-linking or by irradiation.

When the coating composition of the present invention is cross linkedwith a polyol or a peroxide, a suitable cross-linking agent is added tothe composition.

In case of the amine cross-linking, a primary or secondary polyamine isused as the cross-linking agent.

In case of the polyol cross-linking, a combination of a polyol (e.g.bisphenol-A and bisphenol-AF) and a cross-linking accelerator (e.g.calcium hydroxide, barium hydroxide, guanidine and imidazole) ispreferably used.

In case of the peroxide cross-linking, a peroxide having a half-lifetemperature in a period of 10 hours of 100° C. or higher (e.g. dicumylperoxide, cumyl hydroperoxide and tert.-butyl peroxide) is used. Theperoxide is usually used in combination with a polyfunctional compound(e.g. triallyl isocyanurate, triallylene cyanurate and diallylphthalate).

When the powder coating composition contains the cross-linking agent andits cross-linking rate is too high, a leveling property of a coated filmis deteriorated so that coating performance of the composition isdecreased. On the contrary, in case of too low a cross-linking rate, asagging of the composition is easily formed. Then, the cross-linkingagent having an induction time of 5 to 30 minutes at least at 100° C. orthe cross-linking agent which does not induce cross-linking at atemperature of 100° to 130° C. but quickly induces cross-linking whenthe temperature is increased by 20° to 50° C. From this standpoint, theperoxide cross-linking is most preferred, and the polyol cross-linkingand the amine cross-linking are less preferred in this order.

The radiation cross-linking is effected after leveling or on a processedarticle.

When a thermoplastic fluororubber is used as the fluororubber, nocross-linking is necessary. The thermoplastic fluororubber is heated toa plastic state, coated on a substrate and cooled to form a hardenedfilm.

As the adhesion-preventing material, any of the conventional additivessuch as a plasticizer, a bulk filler, a coloring agent, an acid acceptorand the like may be used.

Examples of the plasticizer are calcium stearate, magnesium stearate,dioctyl phthalate, dicresyl phthalate and the like. Examples of the bulkfiller are inorganic salts (e.g. barium sulfate and calcium carbonate)and carbonaceous materials (e.g. carbon black and graphite). Examples ofthe coloring agent are metal oxides such as titanium oxide, iron oxideand molybdenum oxide. Examples of the acid acceptor are magnesium oxide,calcium oxide and lead oxide.

The powder of the coating composition of the present inventionpreferably has an average particle size of 10 to 2,000 μm. When theaverage particle size is larger than 2,000 μm, the coated film has poorsurface properties (surface smoothness). When it is less than 10 μm, thecontent of the adhesion-preventing material increases so that physicalproperties of the coated film (e.g. rubbery elasticity and chemicalresistance) are deteriorated.

In case of the powder coating composition to be used in electrostaticpowder coating, the powder has an average particle size of 10 to 150 μm,preferably 20 to 100 μm. In case of a fluidized bed coating, the powderhas an average particle size of 100 to 300 μm. In case of rotolining orrotomolding, the powder has an average particle size of 150 to 2,000 μm,preferably 150 to 1,000 μm.

The average particle size of the powder is determined according to ASTMD-1457 and D-1921-63 as follows:

An amount of the powder is sieved by a ro-tap shaker through a series ofpiled up sieves having meshes which differ from one another. From themesh size and the cumulative remaining percentage, which is calculatedfrom a weight of the powder remaining on each sieve, the averageparticle size of the powder is determined by means of a logarithmicnormal probability paper.

The powder preferably has an apparent density of at least 0.2 g/cc. Whenthe apparent density is less than 0.2 g/cc, deaeration during theformation of the film is difficult, which causes foaming. There is nospecific upper limit of the apparent density. Generally, it does notexceed 1.48 g/cc. The apparent density of the powder is measuredaccording to JIS K 6891.

The powder coating composition of the present invention can be preparedby various methods. For example, the fluororubber and optionally thefiller and the cross-linking agent are homogeneously mixed by a mixingapparatus such as a calender roll and a kneader and comminuted by asuitable apparatus such as a cutter mill, a jet mill and a hammer mill.The powder can be prepared by comminuting the fluororubber in a solventwhich does not solubilize but swells the fluororubber. Also, the powdercan be prepared by spray drying an aqueous dispersion or a non-aqueoussolution of the fluororubber. Alternatively, the powder can be preparedby coagulating the fluororubber from an aqueous dispersion of thefluororubber and separating and fluidization drying the coagulatedfluororubber. Preferably, the powder is prepared by freezepulverization.

The adhesion-preventing material is applied on the comminutedfluororubber by mixing the material with a conventional apparatus suchas a twin-cylinder mixer and a spiral agitator, a marine propellerstirrer.

The amount of the adhesion-preventing material is from 0.05 to 10% byweight based on the weight of the fluororubber.

Processing conditions for the powder coating composition of the presentinvention varies with the application methods. Generally, the followingconditions are employed:

In the case of the electrostatic powder coating, the powder coatingcomposition is electrostatically deposited on a substrate while applyinga voltage of 10 to 80 KV, leveled at a temperature of 60° to 130° C. andthen cross linked at a temperature of 140° to 200° C. to form ahomogeneous coated film. In the case of fluidized bed coating, after thesubstrate to be coated is pre-heated to a temperature of 100° to 130°C., the powder coating composition is melt coated on the heatedsubstrate followed by leveling and cross-linking as in case of theelectrostatic powder coating. In the case of the rotolining orrotomolding, the powder coating composition is filled in a mold ordeposited on a substrate and gradually heated to a cross-linkingtemperature to effect leveling and cross-linking while rotating the moldor the substrate.

The thickness of the coated film or the molded article which is formedfrom the powder coating composition of the present invention is usuallyfrom 50 μm to 10 mm. If desired, a film having a thickness of 10 mm orlarger may be formed.

Since the applied composition is to be leveled during processing,preferably the fluororubber to be used has a Mooney viscosity ML₁₊₁₀(measured after one minute holding and then ten minutes rotation) of 200or less, preferably 150 or less at 100° C. The Mooney viscosity ismeasured according to JIS K 6300. When the fluororubber has a Mooneyviscosity (ML₁₊₁₀) larger than 200, the balance between the leveling andthe cross-linking is lost when the temperature is raised to effect theleveling so that the leveling of the coated composition is deteriorated.In view of the sag problem or storage stability of the coatingcomposition, the fluororubber preferably has a Mooney viscosity ML₁₊₁₀of 20 or larger at 60° C.

Preferably, the powder coating composition of the present invention hasan angle of repose of 20° to 60°. The angle of repose is defined as anangle between a horizontal plane and a ridge of piled powder which isformed by dropping an amount, usually about 100 ml of the powder from aheight of 5 cm on a sheet of paper. When the angle of repose exceeds60°, irregularity in the coated film tends to appear. When said angle isless than 20°, an excess of adhesion-preventing material is trapped bythe fluororubber and as a result, would adversely affect the propertiesof the coating composition.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be explained in greater detail by thefollowing Examples. The "parts" and "%" are by weight unless otherwiseindicated.

The compounds of the fluororubber used in Examples have followingcomposition:

    ______________________________________                                        Component               Parts                                                 ______________________________________                                        Compound 1:                                                                   Daiel G-902             100                                                   (Vinylidene fluoride/hexafluoro-                                              propylene/tetrafluoroethylene copolymer                                       manufactured by Daikin Industries, Ltd.)                                      Medium thermal carbon   20                                                    Perhexa-2,5-B           1.5                                                   (Peroxide compound manufactured by                                            Nippon Oil & Fat Co., Ltd.)                                                   Triallyl isocyanurate   4                                                     [ML.sub.1+10 (100° C.) = 30]                                           Compound 2:                                                                   Daiel G-704             100                                                   (Vinylidene fluoride/hexafluoro-                                              propylene copolymer manufactured by                                           Daikin Industries, Ltd.)                                                      Medium thermal carbon   20                                                    MgO                     3                                                     Ca(OH).sub.2            6                                                     [ML.sub.1+16 (100° C.) = 80]                                           Compound 3:                                                                   Daiel Perfluor GA-50    100                                                   (A peroxide cross-linking composition of                                      tetrafluoroethylene/perfluoro(alkyl                                           vinyl ether) copolymer manufactured by                                        Daikin Industries, Ltd.)                                                      [ML.sub.1+10 (100° C.) = 10]                                           Compound 4:                                                                   Tetrafluoroethylene/propylene copolymer                                                               100                                                   (Molar ratio of 55:45)                                                        Medium-thermal carbon   20                                                    Perhexa-2,5-B           1.5                                                   Triallyl isocyanurate   4                                                     [ML.sub.1+10 (100° C.) = 30]                                           ______________________________________                                    

Example 1

The fluororubber composition of Compound 1 was pulverized with a cuttermill at -80° C. and the surfaces of the powder particles of thepulverized fluororubber composition were coated with 0.5% of bariumsulfate based on the weight of the fluororubber composition. Then, thepowder was sieved to obtain a powder composition having an averageparticle size of 50 μm and an apparent density of 0.45 g/cc.

The powder was electrostatically coated on a steel plate (100 mm×150mm×2 mm) at an applied voltage of 40 KV and then heated at 100° C. for15 minutes. The electrostatic powder coating and heating were repeatedtwo more times and the coated steel plate was finally baked in the airat 100° C. for 30 minutes and in a nitrogen stream at 160° C. for 60minutes to cure the fluororubber. Thereby, a smooth coated film of about5 mm in thickness containing no bubbles was formed.

Example 2

The fluororubber composition of Compound 2 was pulverized and sieved inthe same manner as in Example 1 to obtain a powder composition having anaverage particle size of 40 μm and an apparent density of 0.42 g/cc.

The powder was electrostatically coated on a steel plate (100 mm×150mm×2 mm) at an applied voltage of 40 KV and then heated at 120° C. for15 minutes. The electrostatic powder coating and heating were repeatedthree more times and the coated steel plate was finally baked in the airat 120° C. for 30 minutes and in a nitrogen stream at 160° C. for 60minutes to cure the fluororubber. Thereby, a homogeneous coated film ofabout 4 mm in thickness was formed.

Example 3

The fluororubber composition of Compound 3 was pulverized and sieved at-120° C. in the same manner as in Example 1 to obtain a powdercomposition having an average particle size of 55 μm and an apparentdensity of 0.53 g/cc.

In the same manner as in Example 1 but changing the leveling temperatureto 80° C. and using the above powder, a homogeneous coated film of about5 mm in thickness was formed on the steel plate.

Example 4

In the same manner as in Example 1 but using the fluororubbercomposition of Compound 4, the powder having an average particle size of45 μm was obtained. From this powder, a homogeneous coated film of about4.5 mm in thickness was formed on the steel plate.

Example 5

Among the sieved powder obtained in Example 1, that having the particlesize of 100 to 200 μm and the apparent density of 0.65 g/cc was chargedin a fluidized bed coating tank and fluidized with air blow. In thefluidized powder, a steel plate (100 mm×150 mm×5 mm) which was preheatedat 120° C. was dipped for 1 minute. After blowing off excess powder withan air gun, the coated steel plate was baked at 120° C. for 10 minutes.The blowing off with the air gun and baking of the dipped coating wererepeated two more times. Finally, the coated composition was leveled at120° C. for 20 minutes and then cured in the nitrogen stream at 160° C.for 60 minutes. Thereby, a smooth coated film of about 5.5 mm inthickness containing no bubbles was produced.

Example 6

Among the sieved powder in Example 1, that having the particle size of200 to 1,000 μm and the apparent density of 0.90 g/cc (1,800 g) wascharged in a mold having an interior space of 3 liters the inner wall ofwhich was coated with a silicone type mold release agent and molded in anitrogen stream while double axially rotating the mold. During molding,the temperature was raised at a rate of 5° C./min. to 180° C. and keptat that temperature for 20 minutes followed by cooling. Then, the moldedarticle was removed from the mold to obtain a 3 liter container having awall thickness of about 10 mm.

Physical properties (tensile strength, elongation, tear strength andhardness) and chemical resistance of the films or the molded articleproduced in Examples 1-6 were measured.

The chemical resistance was evaluated by measuring a volume increaseafter dipping the films or the article in each chemical in comparisonwith the volume before dipping. The volume increase of less than 5% ismarked "O", and that of 10% or larger is marked "X".

The results are shown in Table.

                  TABLE                                                           ______________________________________                                                     Example No.                                                                   1    2      3      4    5    6                                   ______________________________________                                        Physical properties                                                           Tensile strength (MPa)                                                                        18.5   15.0   11.0                                                                               12.0                                                                               18.0                                                                               18.5                             Elongation (%) 350    290    550  200  360  330                               Tear strength (KN/m)                                                                          20     21     18   15   20   19                               Hardness        71     71     69   76   71   72                               Chemical resistance                                                           Sulfuric acid (90%)                                                                          ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                     (80° C. × 70 hrs)                                                Sodium hydroxide (20%)                                                                       ◯                                                                        ◯                                                                        ◯                                                                      ◯                                                                      ◯                                                                      ◯                     (80° C. × 70 hrs)                                                Gasoline       ◯                                                                        ◯                                                                        ◯                                                                      X    ◯                                                                      ◯                     (40° C. × 70 hrs)                                                Methanol       ◯                                                                        X      ◯                                                                      X    ◯                                                                      ◯                     (40° C. × 70 hrs)                                                ______________________________________                                    

Example 7

As a fluororubber powder, the thermoplastic fluororubber DaielThermoplastic T-630 (manufactured by Daikin Industries, Ltd., ML₁₊₁₀=180) was used and treated with calcium stearate to obtain a powdercomposition having a particle size of 20 to 100 μm and an apparentdensity of 0.47 g/cc. In the same manner as in Example 1, the obtainedpowder was electrostatically coated on the steel plate and heated at200° C. repeatedly. Then, the coated film was leveled at 190° C. for 20minutes to obtain a homogeneous film of 3 mm in thickness.

What is claimed is:
 1. A method for coating an article with a fluororubber comprising the steps of:applying on a substrate a fluororubber-containing powdery composition, which is prepared by freeze pulverization, comprising 0.05 to 10% by weight based on the weight of the fluororubber of at least one adhesion-preventing material selected from the group consisting of a plasticizer, a bulk filler, a coloring agent and an acid acceptor and optionally a cross-linking agent, which composition has a particle size of 10 to 2,000 μm and an apparent density of at least 0.2 g/cc and an angle of repose of 20° to 60° C., wherein said fluororubber is selected from the group consisting of vinylidene fluoride/hexafluoropropylene copolymers, vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymers, tetrafluoroethylene/propylene copolymers, hexafluoropropylene/ethylene copolymers, perfluoro(alkyl vinyl ether)/olefin copolymers, and fluorosilicone polymers; and heating said composition at 60° to 130° C. to effect leveling of said composition, and optionally heating said composition at 140° to 200° C. to crosslink the fluororubber, wherein said substrate is not deformed at the highest temperature of the above leveling and crosslinking temperatures.
 2. The method according to claim 1, wherein the fluororubber-containing composition is applied on the substrate by an electrostatic powder coating method.
 3. The method according to claim 1, wherein the fluororubber-containing composition is applied on the substrate by fluidized bed coating.
 4. The method according to claim 1, wherein the fluororubber-containing composition is applied on the substrate by rotolining or rotomolding.
 5. The method according to claim 1, wherein the fluororubber has a glass transition temperature of not higher than 10° C.
 6. The method according to claim 1, wherein the adhesion-preventing material is selected from a plasticizer, a bulk filler, a coloring agent or an acid acceptor.
 7. The method according to claim 6, wherein the adhesion-preventing material is a plasticizer selected from calcium stearate, magnesium stearate, dioctyl phthalate or dicresyl phthalate.
 8. The method according to claim 6, wherein the adhesion-preventing material is a bulk filler selected from an inorganic salt or a carbonaceous material.
 9. The method according to claim 6, wherein the adhesion-preventing material is a bulk filler selected from barium sulfate, calcium carbonate, carbon black or graphite.
 10. The method according to claim 6, wherein the adhesion-preventing material is a metal oxide coloring agent.
 11. The method according to claim 6, wherein the adhesion-preventing material is a coloring agent selected from titanium oxide, iron oxide or molybdenum oxide.
 12. The method according to claim 6, wherein the adhesion-preventing material is an acid acceptor selected from magnesium oxide, calcium oxide or lead oxide. 